As the new uBlox Zed-F9P GPS is starting to become available I thought it would be worthwhile to do some side by side testing with the previous generation uBlox M8P. This testing is without external corrections, so both GPS modules are running as standalone GPS units.
The key change with the F9P is that it is dual-band when paired with a dual-band antenna. That allows is to estimate ionospheric corrections much more accurately than can be done with a single band GPS module like the M8P.
Test Setup
GPS modules used were:
- Drotek UBlox Zed-F9P with dual-band antenna
- Hex Here+ GPS (based on uBlox M8P)
The two modules were connected to the same Hex CubeBlack running ArduPilot, with the M8P GPS as GPS1 and the F9P as GPS2.
Note that the external antenna, long antenna cable and magnetic base of the F9P setup make it impractical for most small aircraft, but modules with integrated multi-band patch antennas are now starting to become available. The first of those is here:
I don’t have one of those yet (it wasn’t available when I ordered the Drotek unit), so I tested with the one I had. I look forward to other vendors releasing more F9P based GPS modules.
Scenarios
Three scenarios were tested
- a extremely poor location looking out the window of a house with a metal roof, looking over another house. Clear view of the sky is only available over about 20 degrees vertically. This represents extreme multi-pathing.
- a good location with clear sky view
- a medium location with medium density tree cover and a nearby house
All 3 locations were in Canberra, Australia.
Even though these GPS modules are capable of RTK fixes a source of corrections was not supplied. The aim of these tests was to see if the dual band and improved processing in the F9P would produce less variance in position and altitude without external corrections.
Location 1:
The first location was with extreme multi-pathing. The antennas were up against a window in a house with a metal roof and metal eaves, looking over another house.
First we show the altitude reading in meters over a 12 hour period:
And now the horizontal error in meters:
Location 2:
This location was at the CMAC flying field, with clear view of the sky in all directions. Altitude in meters over 3 hours:
and horizontal error in meters:
Location 3
This location was in the backyard of my house, with medium density tree cover. Altitude error in meters over 3 hours:
and horizontal error in meters:
Conclusion
The F9P with a dual-band antenna provides a very large reduction in both altitude drift and horizontal drift over a single band M8P. Altitude drift is still high enough with the F9P to make using a rangefinder or other altitude source for landing worthwhile, but mission accuracy should be a lot better.
The F9P reported approximately twice as many satellites as the M8P in each situation, with up to 30 satellites tracked with a clear view of the sky. It seems likely that it was the same satellites as the M8P with each reported once for each band (L1 and L2).
